Story highlights

For anti-Trump forces, Tuesday is single most important day of election, Sexton says

He says only two options left for them: an anti-Trump war of attrition or a Cruz miracle

Buck Sexton is a political commentator for CNN and host of "The Buck Sexton Show" on TheBlaze. He was previously a CIA counterterrorism analyst. The opinions expressed in this commentary are his.

(CNN)The moment that many Republicans thought -- even hoped -- was impossible is now upon us: By Tuesday's end, Donald Trump could become the de facto presidential nominee of their party. Whether he does will largely come down to the will of voters in two states: Ohio and Florida.

Based on the most recent polls, a Trump victory appears to be a tossup in Ohio but a shoo-in for Florida, where he leads by high double digits. Every realistic scenario shows Trump building on his lead Tuesday, and perhaps extending it beyond the reach of his opponents entirely. The GOP's day of reckoning has come.

It's not just delegate math but campaign momentum that hangs in the balance. If Trump finishes first in the winner-take-all states of Ohio and Florida, he will be more than halfway to the 1,237 delegates he needs to lock up the GOP nomination. Just as importantly, he will have prevailed in two of the most essential swing states for the upcoming general election.

The narrative of Trump as the only legitimate nominee and best contender to beat Hillary Clinton (or Bernie Sanders) will become gospel among his supporters and increasingly difficult for his opponents to counter.

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Photos:Trump campaign: 11 outrageous quotes

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Even the rosiest scenarios for the anti-Trump forces now look pretty bleak over the long run. If Trump wins Florida but loses Ohio, he will maintain a substantial delegate lead with the likelihood of strong primary finishes in delegate-rich states such as New York, California and New Jersey ahead. And if he fails to win either Ohio or Florida, which is highly unlikely, he will still get enough delegates in other contests to remain in first place and face a fractured field of candidates going forward -- as Marco Rubio and John Kasich will be almost certain to stay in it if they have big hometown wins.

This is the painful reality for the GOP establishment: No matter what the outcome of Tuesday's winner-take-all primaries, it already has no good options to stop The Donald. Every other candidate's pathway to victory looks long and ugly. And even if Kasich manages to hold his home turf in Ohio, Rubio pulls out a miracle in Florida and Sen. Ted Cruz has strong finishes in Missouri, Illinois and North Carolina, the strategies to stop Trump are long shots at best.

For those conservatives, libertarians and mainstream Republicans who refuse to give in and want to beat Trump, there are really only two options left, and neither will be easy:

Photos:John Kasich's political career

Ohio Gov. John Kasich speaks at the First in the Nation Republican Leadership Summit on April 18, 2015, in Nashua, New Hampshire. The summit was attended by all the 2016 Republican candidates as well as those eying a run for the nomination. Click through for more on the political career of Kasich:

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Photos:John Kasich's political career

Kasich speaks during the Republican National Convention at the Tampa Bay Times Forum on August 28, 2012, in Florida.

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Photos:John Kasich's political career

Kasich, left, and then-Republican presidential candidate Mitt Romney talk with students during a roundtable discussion at Otterbein University on April 27, 2012 in Westerville, Ohio. Romney eventually won the 2012 GOP presidential nomination.

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Photos:John Kasich's political career

Kasich, left, President Barack Obama, center, and Republican House Speaker John Boehner play the first hole of a golf game on June 18, 2011, at Andrews Air Force Base in Maryland.

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Photos:John Kasich's political career

Kasich listens to Obama speak during a bipartisan meeting of governors hosted by the President and Vice President Joe Biden in the State Dining Room of the White House on February 28, 2010.

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Photos:John Kasich's political career

Before officially taking office as governor of Ohio, Kasich talks with reporters after meeting with House and Senate Republican leaders at the U.S. Capitol on December 1, 2010. The GOP leaders talked about ways to create jobs, cut spending and repeal the health care law.

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Photos:John Kasich's political career

Kasich, right, then a member of the U.S. House, sports "Bush" baseball caps with Texas Gov. George W. Bush on July 14, 1999. The two lawmakers held a news conference at the Ronald Reagan International Trade Center in Washington during Bush's presidential run. Kasich had previously announced that he was withdrawing from the 2000 presidential race and endorsed Bush.

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Photos:John Kasich's political career

U.S. Rep. Kasich delivers a speech in the Watergate complex in Washington on July 9, 1999, during the College Republican National Committee 53rd Biennial Convention. Other speakers included Republican 2000 presidential hopefuls such as Gary Bauer and Elizabeth Dole.

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Photos:John Kasich's political career

During a U.S. government shutdown, Kasich, left, and Sen. Pete Domenici, R-New Mexico, speak to reporters outside the White House on January 6, 1996.

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Photos:John Kasich's political career

Kasich shows a videotape of President Bill Clinton speaking during a news conference on Capitol Hill on December 16, 1995. Kasich was chairman of the House Budget Committee at the time and disputed Clinton's position on the budget.

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Photos:John Kasich's political career

Kasich, center, shows a thank you note in the form of a check to Speaker of the House Newt Gingrich, left, and Senate Majority Leader Robert Dole (right) on November 17, 1995, in Washington. They were soon engaged in bruising battles with President Bill Clinton over the federal budget.

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1. The anti-Trump war of attrition: In this scenario, Rubio, Cruz and Kasich stay in, regardless of their March 15 showings, and all throw their efforts into preventing Trump from getting to the magic 1,237 number. With Rubio reportedly telling his supporters to vote for Kasich in Ohio to prevent a Trump victory there, some elements of this strategy already appear to be in play.

Trump's opponents would continue to work in concert to deny him a majority, and the whole contest would drag on into the summer, consuming resources while the Democratic nominee focuses on fund-raising and general election campaigning.

Eventually, this approach lands the GOP at a brokered convention in Cleveland in July. At that point, as the theory goes, rule changes, political horse-trading and backroom machinations could pull the nomination away from Trump the front-runner and hand it to one of his rivals, or even some outsider brought in by the GOP elites.

Politically speaking, such moves would be burning down the village to save it. They may preserve the GOP's brand for the next go-round, but in the short run, they would almost certainly hand the White House to the Democrats. Trump's supporters would see such convention collusion as yet another betrayal at the hands of the very GOP overlords against whom they have revolted.

2. The Cruz miracle: The senator from Texas has been the only candidate to beat Trump in head-to-head competition in numerous states with substantial delegates in play. Other contests looming ahead, such as Arizona and Utah, look strong for Cruz. And in national polling, head-to-head, Cruz beats Trump by a wide margin. Cruz can win in a fair fight against Trump, but he has to get there first.

The problem Cruz has to overcome is a still too-crowded field and a campaign clock that is quickly running out. He needs Rubio and Kasich to clear the way and allow for the GOP contest to become a two-man race, and soon. The only way that happens is if Rubio loses, and somehow Kasich is convinced that, no matter what happens in Ohio, he has no realistic shot at the nomination. And keep in mind, with a win in Florida adding wind into his campaign's sails, Trump would be even harder for Cruz to catch.

Photos:Moments from Ted Cruz's career

U.S. Sen. Ted Cruz, a Republican presidential candidate, speaks during a campaign rally at the Indiana State Fairgrounds on Monday, May 2.

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Photos:Moments from Ted Cruz's career

Cruz holds up the hand of Carly Fiorina at a campaign rally in Indianapolis on Wednesday, April 27. Cruz named Fiorina, a former presidential candidate, as his running mate.

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Photos:Moments from Ted Cruz's career

Cruz celebrates his Wisconsin primary win with his wife, Heidi, and Gov. Scott Walker in Milwaukee on Tuesday, April 5. Walker endorsed Cruz for the presidency.

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Photos:Moments from Ted Cruz's career

With his wife by his side, Cruz tours the Dane Manufacturing facility before speaking to workers in Dane, Wisconsin, on Thursday, March 24.

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Photos:Moments from Ted Cruz's career

Cruz speaks during the CNN Republican debate in Miami on Thursday, March 10.

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Photos:Moments from Ted Cruz's career

Cruz and his wife wave to the crowd at Liberty University after he announced his presidential candidacy in Lynchburg, Virginia, on March 23, 2015.

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Photos:Moments from Ted Cruz's career

Cruz speaks during the 2013 NRA Annual Meeting and Exhibits at the George R. Brown Convention Center on May 3, 2013, in Houston, Texas.

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Photos:Moments from Ted Cruz's career

Cruz poses with his wife, Heidi, and his daughters Caroline and Catherine.

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Photos:Moments from Ted Cruz's career

Cruz (left) fields questions from Bruce Rastetter at the Iowa Ag Summit on March 7, 2015, in Des Moines, Iowa. The event allows the invited speakers, many of whom are potential 2016 Republican presidential hopefuls, to outline their views on agricultural issue.

Sen. Patrick Leahy (right) escorts Loretta Lynch back from a lunch break as Cruz (left) sits nearby during her confirmation hearing before the Senate Judiciary Committee January 28, 2015, on Capitol Hill.

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Photos:Moments from Ted Cruz's career

Cruz greets supporters at the South Carolina Tea Party Coalition convention on January 18, 2015, in Myrtle Beach, South Carolina. A variety of conservative presidential hopefuls spoke at the gathering on the second day of a three-day event.

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Photos:Moments from Ted Cruz's career

Cruz (left) and then-Texas Governor Rick Perry stand together during a press conference at the front gate of Fort Hood about Iraq war veteran, Ivan Lopez, who killed three and wounded 16 before taking his own life on April 4, 2014, in Fort Hood, Texas.

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Photos:Moments from Ted Cruz's career

(Left to right) Sen. Lindsey Graham, Sen. John McCain and Cruz listen as President Barack Obama delivers the State of the Union address on January 28, 2014, in Washington, D.C.

Cruz listens to testimony during a Senate Judiciary Committee hearing on April 22, 2013, in Washington, D.C.

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Photos:Moments from Ted Cruz's career

Cruz holds a news conference to announce the plan to defund Obamacare on March 13, 2013.

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Photos:Moments from Ted Cruz's career

Cruz speaks at the CPAC on March 6, 2014, in National Harbor, Maryland.

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Photos:Moments from Ted Cruz's career

Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks at the 'Patriots for Romney-Ryan Reception' on August 29, 2012, in Tampa, Florida.

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Photos:Moments from Ted Cruz's career

Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks during the Republican National Convention at the Tampa Bay Times Forum on August 28, 2012.

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Photos:Moments from Ted Cruz's career

Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks during the Republican National Convention in 2012.

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The GOP campaign won't officially be over by Wednesday morning, and in a political season that has made a mockery of the conventional wisdom, anything seems possible. But if Trump racks up a few more big wins Tuesday, the nomination battle could be as good as over, and the GOP better get ready for a new kind of March Madness.